HESCO ProactiView on Level Measurement

Knowing the level of a material in your process is a seemingly simple and straightforward measure, and a fundamental piece of data necessary to manage a complex process system. Yet, there are several approaches to gathering this simple piece of data, and just as many “expert” opinions as to which is the best. Should you use point level or continuous measure? Should you use contacting or non-contacting instruments? The most information and effectiveness comes from non-contacting, continuous level measurement, which leads to the hottest question of all today: Should you use ultrasonic or radar equipment?

Head spinning yet? Don’t listen to anyone who wants to tell you that either ultrasonic or radar equipment is always better than the other … read on for a primer of the similarities and differences of these complimentary technologies.

HESCO can then help you evaluate these differences and help select the most appropriate technology. The information below will get you started, and we stand by, ready and able to go more in depth (pun intended) when you call us!

Contents

Point versus Continuous Level Measurement
Contacting versus Non-Contacting Level Measurement
Basics of Non-Contact, Continuous Level Measurement
Ultrasonic Measurement
Radar Measurement
Technology Comparison
Follow Up with HESCO Regarding Level Measurement

Point versus Continuous Level Measurement

Today, level sensors generally are divided into two categories; point level and continuous level.  Point level indicates when material is above, below, or at a specific point.  Depending upon the different level positions of the device within a vessel, one can determine if the vessel is full, empty, or somewhere in between. Examples include float switches, capacitance probes and rotary paddles.

Continuous level measurement constantly monitors the level between two discrete points in the vessel, usually full and empty.  Both ultrasonic and radar participate in the point level and the continuous level categories, but most often both are employed for continuous measurement.

Contacting versus Non-Contacting Level Measurement

Within the two categories of point level or continuous level there are two types of instruments; contacting and non-contacting.  A “contacting” instrument is in direct contact with the material being measured.  This can cause problems if the material contains solids that can stick or hang up on the sensor, inhibiting its ability to accurately measure.  A “non-contacting” instrument is mounted well above the material to be measured and uses “Time of Flight” theory to indirectly calculate the height of a material surface.  While many technologies have their place in level measurement, non-contacting level measurement has many advantages in that they generally require less maintenance and are self-calibrating

From here forward, we will focus on non-contacting, continuous level measurement by radar and ultrasonic equipment. 

Basics of Non-Contact, Continuous Level Measurement

Non-contacting, “Time of Flight” devices operate by emitting either sound energy, in the case of an ultrasonic device; or light energy, in the case of a radar device; from a transducer that is mounted above the material to be measured. In both cases, this energy is directed toward the surface of the material to be measured, and reflected back toward the transducer.  These devices detect and record the time it takes for the energy wave to travel from the face of the transducer to the material surface and back again.  The distance that the energy wave traveled is then calculated simply by multiplying ½ of this time by the known velocity of the energy wave in the given medium (typically ambient air).  The level of the material in the vessel is further calculated by subtracting this derived distance from the known height of the transducer from the zero or “empty distance.”
 
Ultrasonic Measurement

Non-contact, noninvasive level measurement was non-existent prior to the development of Ultrasonic Level Technology.  The advantages of the technology were ease of use and low cost of installation.  Further, they were not affected by density or conductivity, and the products were virtually maintenance free.  The initial market reception was great and ultrasonics remain widely accepted today. 

However, some limitations to the technology were quickly realized.  Solid materials having steep angles of repose, long ranges in dusty applications, and air turbulence tend to absorb and/or diffuse the sound energy making it difficult to detect the material surface.  Temperature variations change the speed of sound making it necessary to make adjustments to the formula utilized to calculate the level.  Finally, internal vessel structures also interfered with the signal readings and may cause false readings. 

Radar Measurement

When radar was introduced as a competing, non-contacting technology, many industry experts predicted the doom of ultrasonics.  Radar was considered to be a superior technology that could perform any level measurement that ultrasonics could, only more reliably.  The euphoria over radar did not last long either.  Experience has shown that radar devices have their own set of limitations.  Generally, the environment in which they are deployed must be well understood.  The material and general arrangement of the vessel can have an impact on radar performance.

Radar measurement devices fire microwaves towards the water surface, which travel in straight lines at constant speed, regardless of the environmental conditions such as temperature, pressure, or contents of the media.  Since radar does not use a medium to travel in, it will generally be more applicable in environments that adversely affect the speed of sound.  However, in many applications (especially water and general storage) radar does not offer a performance advantage and generally costs 20% to 30% more.  Additionally there are features such as relays, pump control, remote mounted electronics and multi-point units that are not offered with radar, due to the need to mount the control electronics very near the equipment in order to avoid signal losses.

Technology Comparison

The following table gives a starting point for considering when each technology might be more appropriate, by describing features or conditions that a situation might require.  Again, there are many variables, and this table should not be taken as a single means of decision.

Follow up with HESCO regarding Level Measurement

HESCO, along with its partner, Siemens Process Instrumentation, is uniquely qualified to help you determine which technology is right for you. Siemens offers a complete line of Level Measurement instrumentation, as well as pressure, temperature, and flow instrumentation. Please visit our web-based ProactiView on Level Measurement to download a PDF version of the Siemens Level Technology Selection Guide. As always, we are eager to assist you. If you have a level measurement application or are in need of process instrumentation advice, please contact us directly.

There are two immediate ways that you can contact us:

1.  Call Kevin Livingston or Glenn Hummel at 586-978-7200 and mention that you are following up on the ProactiView on Level Measurement. We will be happy to discuss with you the details of your specific application and begin to help you choose the right equipment.

2. Email Kevin Livingston or Glenn Hummel directly.  Be sure to include your name, contact information, and brief description of your situation, needs and questions. We can attach the Siemens Level Technology Selection Guide upon request.